The present invention relates to a multiple cylinder dryer of a paper machine. More particularly, the invention relates to an air-directing device for a multiple cylinder dryer of a paper machine, which device introduces air to the dryer and includes air-blowing members.
Regarding the technology associated with the invention, reference is made to SE patent No. 67,305, SE patent No. 321,408, CA patent No. 810,896, U.S. Pat. No. 3,283,415, British Pat. No. 927,564 and FI patent No. 45,884.
The present invention touches on the procedure in a multiple cylinder dryer disclosed in Finnish patent application No. 803,720 of the inventor which issued as Finnish Pat. No. 62,693 on Feb. 10, 1983, wherein the dryer comprises a number of heatable cylinders and potentially equivalent rollers. A drying fabric is used in the dryer in support of which drying fabric a web goes from one cylinder and/or roller to another in closed conduction. The cyclic travel of the fabric is so arranged that part of the drying cylinders and/or rollers remain within the loop of the fabric and part thereof outside the loop. One or more supporting fabrics are used in the dryer to carry the web on the cylinders and/or rollers. The supporting fabric are guided by guide rollers. In the dryer, elongated pockets are defined in conjunction with the cylinders by the web runs supported by the drying fabric, by the free surfaces of the drying cylinders and by the runs of the supporting fabric.
The aforedescribed procedure disclosed in said Finnish patent application is mainly characterized in that of the aforementioned pockets in which the web resides on that side of the fabric which faces the pocket have been rendered under atmospheric pressure, at least in the initial part of the dryer, as viewed in the direction of travel of the web, and under a pressure higher than in the adjacent pockets, so that the differential pressure in question will urge the web against the felt with a view to stabilizing the running of the web.
It is known in the art to utilize various types of air conditioning apparatus, blow tubes, etc. in the drying section of a paper machine. These have been used in connection with twin-wire conduction for regulating the humidity of air in the pockets. They influence the pressure level in certain instances, but since this influence is not directional in any way, and since the pressure level is the same in the pockets on both sides of the web, it has not been possible thereby to effect the behavior of the web. As known in the art, more modern apparatus of this type have been located on the non-paper side of the felts, and the older apparatus, when actual felts are used, have been located on the paper side of the fabric or at the ends of the pockets.
In connection with single felt conduction, as known in the art, no actual pocket ventilating devices have been used, as a rule. Replacement air has in some instances been introduced from below the single felt conduction drying section by header beams extending across the machine and usually disposed in the interstices of the transversal structural beams of the building. The structural designs have recently been altered in such a way that the beams are completely omitted. For this reason, separate air introduction tubes are no longer permitted under a single felt drying section. Such tubes might break in the event of a web break. It has not been possible by this procedure to promote web stability.
Blowing from the side in, under the machine, has been utilized in connection with single felt conduction for the purpose of supplying replacement air. The pressure level and the behavior of the web, in the sense implied in the invention, could not be influenced by this expedient, either. On the contrary, this procedure of the prior art has caused instability of the web, particularly in the marginal part.
Although the air-directing device of the invention is primarily intended to be used in connection with single felt conduction, it is appropriate for use more universally.